The present invention relates to a liquid crystal material, more particularly, a liquid crystal display device improved in moving picture quality.
In liquid crystal display devices, an electric field is applied to liquid crystal molecules in a liquid crystal layer inserted between a pair of substrates and a display is given by utilizing a change in the optical properties of the liquid crystals caused by a change in the direction of orientation of the liquid crystal molecules caused by the application of the electric field. Such liquid crystal display devices are commonly used as the display portions of large-sized liquid crystal TV, monitors for PC, medium- or small-sized car navigation systems, mobile phones and PDA. However, a limited time is required for the change in the direction of orientation of the liquid crystal molecules after the application of the electric field. When this time (=response time) is long, the moving picture quality is deteriorated. This is considered a problem. Such a speed of response is affected by a structure chosen as that of a liquid crystal composition constituting the liquid crystal layer. That is, when a liquid crystal composition having a high viscosity is used, the speed of response is low. On the other hand, when a liquid crystal composition having a low viscosity is used, the speed of response is high.
The following patent document 1 and patent document 2 disclose the structures of liquid crystal compositions which are suitable for display devices from the viewpoint of various characteristics required of a liquid crystal display element. In patent document 1, liquid crystal compounds are chosen from the viewpoint of the extension of mesomorphic range, low viscosity, low threshold voltage, high stability and the improvement of compatibility with other liquid crystal compounds, and their chemical structures are listed. In patent document 2, liquid crystal compounds are chosen which improve low-voltage driving, high-precision display, high contrast ratio, high visual angle characteristics, low-temperature response characteristics and the like, and the employment of a liquid crystal composition having a low viscosity is described for improving, in particular, the speed of response.
Patent document 1: Japan Patent No. 3287288
Patent document 2: Japan Patent No. 3178890
The viscosity of the above-mentioned liquid crystal composition has exponential temperature dependence and increases with a lowering of temperature. Therefore, the speed of response of the liquid crystals is also affected by a temperature change. In recent years, liquid crystal display devices such as portable or on-vehicle displays are often used in a circumstance of various temperatures. Accordingly, in order to investigate the moving picture quality of the liquid crystal display devices, it is necessary to investigate not only the improvement of the speed of response but also the temperature dependence of the speed of response. In the above-mentioned patent documents 1 and 2, the most suitable structure of the liquid crystal composition is not chosen in relation to the temperature dependence of the viscosity of the liquid crystal composition.
Such temperature dependence of the viscosity of the liquid crystal composition becomes a more important problem when the employment of the composition in a device is investigated. In a recent liquid crystal display device for TV, the quantity of heat generated by a backlight is increased because the size of the display screen is increased and the brightness is increased by increasing the amount of light emitted by the backlight. Therefore, it is difficult to make the temperature in the liquid crystal display screen uniform, so that a temperature distribution exists in a plane of the liquid crystal layer. In order to use light-emitting diodes (LED) as a backlight in place of a cold-cathode tube, a technique for dimming and controlling a plurality of LEDs as point light sources independently has been proposed. When the technique for thus controlling the backlight in each region is adopted, the following problem is caused: the temperature distribution in a plane of the liquid crystal layer becomes remarkable, so that the speed of response is changed in every region in the display screen in the case of a liquid crystal composition having much temperature dependence, resulting in a deteriorated moving picture display.
The speed of response τ of the device is proportional to the product of the viscosity η of the liquid crystal composition and (the thickness of the liquid crystal layer d)2 when the temperature dependence of the elastic constant is neglected. In a liquid crystal display device having a multi-gap structure in which the thickness of a color filter film is different for pixels for different colors, i.e., red, green and blue, or a semi-transmission type liquid crystal display device in which the thickness values of a liquid crystal layer in a reflection portion and a transmission portion, respectively, are made different in order to adjust optical characteristics in these portions, the speed of response is different in regions different in the thickness of the liquid crystal layer. If the viscosity of the liquid crystal composition changes markedly with temperature, such a liquid crystal display device is disadvantageous in that moving picture characteristics in each region change markedly with temperature.
The above-mentioned problems, i.e., the partial deterioration of the moving picture quality by the temperature non-uniformity in the display screen and the widening of the difference in moving picture quality between regions different in the thickness of the liquid crystal layer during low-temperature operation, are caused by the temperature dependence of the response time, i.e., the temperature dependence of the viscosity of a liquid crystal material. A means for solving these problems has not yet been found. These problems are common to all liquid crystal display devices in which a display is given by the use of the electroopitical characteristics of a liquid crystal material, such as liquid crystal display devices in which an electrode is located on each of upper and lower substrates and an electric field substantially perpendicular to the substrate surface is applied and which are represented by liquid crystal display devices according to the twisted nematic (TN) method, vertical alignment (VA) method or electrically controlled birefringence (ECB) method; and liquid crystal display devices according to in-plane switching (IPS) method, a display method in which an inter digital electrode is located on one of upper and lower substrates, the direction of application of an electric field is made substantially parallel to the in-plane direction of the substrate and the change of the birefringence of liquid crystals is utilized.
An object of the present invention is to attain a good moving picture quality as follows irrespective of a liquid crystal display method such as VA method or IPS method: when the response time of liquid crystals is different in different pixels or nonuniform in one and the same pixel, as in a liquid crystal display device in which the thickness of a liquid crystal layer is different in different pixels or nonuniform in one and the same pixel, a difference in the response time made by a temperature lowering is reduced, or the temperature dependence of the response time is reduced which is observed as a difference in the response time between a high-temperature portion and a low-temperature portion made when the surface temperature of the display screen of the liquid crystal display device is not uniform.